Effect of soil transfer on ectomycorrhiza formation and the survival and growth of conifer seedlings on old, nonreforested clear-cuts

Abstract

Small amounts (150 mL) of soil from established conifer plantations and mature forest were transferred to planting holes on three clear-cuts in southwest Oregon and northern California to enhance mycorrihiza formation. The clear-cuts, 8–27 years old and unsuccessfully reforested, included a range of environmental conditions. At Cedar Camp, a high-elevation (1720 m) southerly slope with sandy soil, transfer of plantation soils increased 1st-year Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedling survival by 50%. Notably, soil from a plantation on a previously burned clear-cut doubled mycorrhiza formation and tripled seedling basal area growth. Soil from mature forest did not improve survival and growth. Less dramatic effects owing to soil transfer were evident on other sites, which were lower in elevation and had clayey soils with greater water-holding capacity, and where woody shrubs had apparently preserved mycorrhizal fungi. At Crazy Peak (1005 m), seedling survival was uniformly good, and soil from a previously burned plantation increased Douglas-fir mycorrhiza formation. At Wood Creek (500 m), soil from a plantation on a previously unburned clear-cut increased mycorrhizal branching on sugar pine (Pinuslambertiana Dougl.) seedlings, but there was no other effect. Results suggest that adequate mycorrhiza formation is critical to seedling growth and survival on cold, droughty sites. Populations of mycorrhizal fungi, and perhaps other beneficial soil biota, decline if reforestation is delayed or other host plants are absent. These declines can be offset by soil transfer from the proper source; in this study, soil from vigorous young plantations.